sets

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;

typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> indexed_set;

int main()
{
    indexed_set s;
    s.insert(2);
    s.insert(3);
    s.insert(7);
    s.insert(9);

    /* 给下标，出数字 */
    auto x = s.find_by_order(2);
    cout << *x << "\n";

    /* 给数字，出下标 */
    cout << s.order_of_key(2) << "\n";
    // 若该元素不存在，s.order_of_key(n) 返回的是传入的数字在s 里本该在的位置
    cout << "Not exists:\n";
    cout << s.order_of_key(6) << "\n";
    cout << s.order_of_key(8);
    return 0;
}

输出：

7
0
Not exists:
2
3